As the function is continuous everywhere it is so in the given interval and thus it is Riemann integrable there. Now subdivide subintervals of equal length , and choose to evaluate the function at the left endpoints of each subinterval (we can do that since we know the functions is Riemann int. and thus we can freely choose the partition ,as long as it length parameter tends to zero when , and the points at each subinterval at which the function's evaluated:

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